The invention relates to a luminescent Eu.sup.2+ activated barium-hexa-aluminate mainly comprising the crystal structure of .beta.'-alumina. The invention also relates to a luminescent screen provided with such an aluminate and to a low-pressure mercury vapour discharge lamp provided with such a screen.
Eu.sup.2+ activated aluminates of Ba and/or Sr and/or Ca defined by the formula Ba.sub.x Sr.sub.y Ca.sub.z Eu.sub.p Al.sub.12 O.sub.19 in which x+y+z+p=1 and 0.001.ltoreq.p.ltoreq.0.1 are known from British Patent Specification No. 1,190,520. The Patent Specification does not state any details on the crystal structure of these efficient luminescent materials. It was, however, known that these materials have a hexagonal crystal structure, which was assumed to relate to the structure of the mineral magnetoplumbite (BaFe.sub.12 O.sub.19). This has indeed proved to be the case for both strontium-hexa-aluminate and for calcium-hexa-aluminate defined by the above-mentioned formula.
However, an Article in Mat. Res. Bull., Vol., 21, 1986, pp. 135-1310, proves that barium-hexa-aluminate occurs in two different phases, namely a barium-poor phase with the crystal structure of .beta.-alumina and a barium-rich phase with the crystal structure of .beta.'-alumina. The structures of the two alumina phases are hexagonal and, although related to that of magnetoplumbite, are clearly different therefrom and from each other. The Article further shows that these barium-hexa-aluminates luminesce efficiently upon activation by Eu.sup.2+ under excitation by short-wave ultraviolet radiation (for example 254 nm). The .beta.-alumina phase exhibits a blue-green emission in a band with a maximum at approximately 476 nm and a half value width of approximately 135 nm, while the .beta.'-alumina phase emits blue radiation in a band with a maximum at approximately 437 nm and a half value width of approximately 55 nm.
According to the articles the two phases in which barium-hexa-aluminate occurs can be described with molecular formulas which are derived from .beta.-alumina, NaAl.sub.11 O.sub.17, and .beta.'-alumina, Na.sub.1.5 Al.sub.10.5 O.sub.16.5, respectively. By replacing 75% of the Na.sup.+ -ions by Ba.sup.2+ -ions and the remaining 25% by O.sup.2 -ions in the formula NaAl.sub.11 O.sub.17 of .beta.-alumina and in the formula Na.sub.1.5 Al.sub.10.5 O.sub.16.5 of .beta.'-alumina, Ba.sub.0.75 Al.sub.11 O.sub.17.25 is obtained for the barium-poor phase with the .beta.-alumina structure and Ba.sub.1.125 Al.sub.10.5 O.sub.16.875 is obtained for the barium-rich phase with .beta.'-alumina structure. In these formulas, the sum of the number of large ions (Ba.sup.2+ +O.sup.2- and Na.sup.+ +O.sup.2-, respectively) per molecule is equal to 18.
The comparatively narrow band emission with a maximum at 437 nm of Eu.sup.2+ activated barium-hexa-aluminate having the .beta.'-alumina structure is very desirable for several practical applications. However, a drawback of this material is that a disturbing quantity of the barium-poor phase with the .beta.-alumina structure is obtained in addition to the desired barium-rich phase with the .beta.'-alumina structure during preparation. Even in the case of starting from the stoichiometric quantities associated with the .beta.'-alumina phase it appears that disturbing side phases may occur. A result thereof is that the barium-hexa-aluminate yields a considerable part of its emission at wavelengths of more than 500 nm. Consequently, suitability of the material for use as a line emitter, for example as the blue component in a low-pressure mercury vapour discharge lamp provided with luminescent materials which emit in three narrow spectral ranges (the so-called three-line lamps) is decreased.
An object of the invention is to provide a luminescent Eu.sup.2+ activated barium-hexa-aluminate which mainly has the crystal structure of .beta.'-alumina and whose emission above 500 nm is considerably suppressed.